FIG. 1 is a diagram showing an example of a circuit configuration of an AC-AC converter described in PLT 1 shown below. In FIGS. 1, 1 and 2 are filter capacitors, 3 and 4 are inductors, 5 to 8 are IGBTs (Insulated Gate Bipolar Transistors) (semiconductor switches that can control the turning on and off of a forward current, and that are always in a state of conduction with respect to reverse current), and 9 and 10 are smoothing capacitors. A circuit configuration wherein the inductor 3 is connected between a series connection point of the IGBTs 5 and 6 of a parallel connection circuit of a series connection circuit of the IGBTs 5 and 6 and a series connection circuit of the capacitors 9 and 10 and one end of an alternating current input, and the other end of the alternating current input is connected to a series connection point of the capacitors 9 and 10, is well known as a high power factor rectifier circuit using a half bridge configuration, and is caused to function as a step-up AC-DC conversion circuit wherein a direct current voltage higher than the peak value of an alternating current input voltage is obtained by switching of the IGBTs 5 and 6.
Also, a circuit configuration wherein the inductor 4 is connected between a series connection point of the IGBTs 7 and 8 of a parallel connection circuit of a series connection circuit of the IGBTs 7 and 8 and the series connection circuit of the capacitors 9 and 10 and one end of an alternating current output, the other end of the alternating current output is connected to the series connection point of the capacitors 9 and 10, and the capacitor 2 is connected between alternating current outputs U and V, is well known as a reverse conversion circuit (inverter) using a half bridge configuration. The circuit is caused to function as a DC-AC conversion circuit wherein a sinusoidal alternating current voltage is obtained from a direct current voltage by switching of the IGBTs 7 and 8.
The AC-AC converter shown in FIG. 1 is used in an application that compensates for voltage fluctuation in an alternating current input, thus supplying a fixed voltage to a load, in an application that supplies uninterrupted power to a load by supplying direct current power to the capacitors 9 and 10 from unshown storage means when an alternating current input is interrupted, and the like.
A bidirectional switch 11 is connected between the series connection point of the IGBTs 5 and 6 and the series connection point of the IGBTs 7 and 8, but firstly, a description will be given hereafter of operations when no bidirectional switch 11 is connected.
An example of an operation when the polarity of an alternating current input current 11 in the high power factor rectifier circuit is positive is as follows.
The current path when the IGBT 6 is turned on is a path from one end Ui of the input terminal through the inductor 3, IGBT 6, and capacitor 10 to the other end Vi of the input terminal, and energy is accumulated in the inductor 3. The current path when the IGBT 6 is turned off is a path from the one end Ui of the input terminal through the inductor 3, the anti-parallel connected diode of the IGBT 5, and the capacitor 9 to the other end Vi of the input terminal, and the energy in the inductor 3 is discharged to the capacitor 9. In this operation, one semiconductor device exists in the current path.
Also, an example of an operation when the polarity of an alternating current output current 12 in the reverse conversion circuit (inverter) is positive is as follows. The current path when the IGBT 7 is turned on is from the capacitor 9 through the IGBT 7, inductor 4, one end U of the alternating current output, the load (not shown), and the other end V of the alternating current output to the capacitor 9. The current path when the IGBT 7 is turned off is from the capacitor 10 through the anti-parallel connected diode of the IGBT 8, the inductor 4, the one end U of the alternating current output, the load (not shown), and the other end V of the alternating current output to the capacitor 10. In this operation, one semiconductor device exists in the current path. The heretofore described operations are such that the current passes through two semiconductor devices until reaching the alternating current output from the alternating current input.
Next, a description will be given hereafter of operations when the bidirectional switch 11 is connected between the series connection point of the IGBTs 5 and 6 and the series connection point of the IGBTs 7 and 8. A first operation is an operation whereby the bidirectional switch 11 is turned on when the IGBTs 5 and 7 are simultaneously turned on, or when the IGBTs 6 and 8 are simultaneously turned on, causing the current to bypass. Furthermore, a second operation is an operation whereby, when fluctuation of the input voltage is within a range tolerated by the load, the switching operation of the IGBTs 5 to 8 is stopped, so that the IGBTs 5 to 8 are in an off-state, and the bidirectional switch 11 is continuously in an on-state. In the first and second operations, as the bidirectional switch 11 is the only semiconductor element in the current path from the alternating current input to the alternating current output, loss is reduced.
When the alternating current input voltage is a voltage within the tolerance value (hereafter called a specified value) of the load connected to the alternating current output, the alternating current input voltage being sent directly to the alternating current output is a method generally implemented in a standby type power supply such as an uninterruptible power system. Meanwhile, the circuit shown in FIG. 1 is such that, when the alternating current input voltage deviates from the specified value, it is possible to restore normal operation delayed by a time (normally a few tens of microseconds) in the region of the switching cycle of the semiconductor element. As disturbance during this period is removed by a filter formed of the capacitors 1 and 2 and inductors 3 and 4, there is an advantage in that, unlike with a standby type power supply, no disturbance occurs in the output.
The examples shown in FIG. 2 are known as examples of a configuration of a bidirectional switch. Also, an example of a configuration of a bidirectional switch different from the bidirectional switches shown in FIG. 2 is shown in FIG. 3, but this will be referred to later. In FIG. 2(a), two reverse blocking IGBTs RB1 and RB2, which have a breakdown voltage with respect to voltage with reverse polarity equivalent to that with respect to forward polarity, are connected in anti-parallel. In FIG. 2(b), circuits provided with reverse breakdown voltage by diodes D1 and D2 being respectively connected in series to normal IGBTs Q1 and Q2, which do not have reverse breakdown voltage, are further connected in anti-parallel. In FIG. 2(c), the IGBT Q1, to which the diode D1 is connected in anti-parallel, and the IGBT Q2, to which the diode D2 is connected in anti-parallel, are connected in anti-series.    PLT 1: JP-A-2006-296098